1. Field of the Invention
The present invention relates to an aligning device between a mask and a photosensitive substrate, which is suitable to a projection type exposure apparatus (stepper or aligner), a proximity type exposure apparatus or the like for transferring onto the photosensitive substrate a pattern of the mask used in the lithography process for production of semiconductor devices or liquid crystal display devices.
2. Related Background Art
Recently, scale-down, projection type exposure apparatus (steppers) using the step and repeat technique have widely been employed in the lithography process as equipment for transferring fine patterns onto photosensitive substrates (semiconductor wafers having photoresist layers formed thereon) with high resolution. In the art of this type steppers, with a higher degree of integration of semiconductor devices, it has been endeavored to shorten the wavelength of light for exposure and develop a projection lens having the increased numerical aperture (N. A.), with the result that a resolvable line width on wafers reaches the order of submicrons (0.5-0.7 .mu.m) at the present time. Transferring patterns at such a high level of resolution requires a pattern of a reticle (synonymous with a mask) and one shot area on a wafer to be aligned with alignment accuracy (usually about 1/5 of the resolvable line width) corresponding to the resolution achieved. As an alignment system for such steppers, there has conventionally been known a TTR (Through The Reticle) technique designed to, for example, simultaneously detect an alignment mark formed around a circuit pattern on the reticle and an alignment mark formed around the shot area on the wafer.
Such an alignment system utilizing the TTR technique comprises the steps of detecting both an alignment mark on the reticle (i.e., a reticle mark) and an alignment mark on the wafer (i.e., a wafer mark) with high precision, determining a shift or deviation in the relative position between the two marks, and moving the reticle or wafer in a fine manner so as to correct the shift. For purpose of imaging a pattern of the reticle on the wafer with high resolving power, a projection optical system is arranged in the prior-art state to be corrected satisfactorily in chromatic aberration against only illumination light for exposure (e.g., a g line of wavelength 436 nm, i line of wavelength 365 nm, or KrF excimer laser beam of wavelength 248 nm). This means that in the alignment system utilizing the TTR technique to detect both the reticle mark and the wafer mark through the projection optical system, the light used in illuminating the marks is limited to the wavelength which is the same as or very close to that of the light for exposure.
A photoresist layer is formed on the wafer surface in the exposure step, and the wafer mark is detected through the photoresist layer at the time of alignment. The photoresist layer has been designed to have a multi-layer photoresist structure or the like which is high in absorptivity and low in transmissivity for the exposure light, in order to permit patterning with higher resolution. This however raises a problem that since the illumination light for alignment is attenuated until reaching the wafer mark and the reflected light (such as the regularly reflected light, the scattered light and the diffracted light) from the wafer mark is also attenuated, the wafer mark cannot be recognized by the alignment system with the sufficient intensity of light, resulting in reduced accuracy of detecting the wafer mark. Further, when the illumination light for alignment is irradiated to the wafer mark to perform alignment, the photoresist layer in that irradiated area is exposed by its very nature and the wafer mark is therefore destroyed while passing through various processes after development. This raises another problem that the mark can no longer be used for alignment in exposure adapted to superpose the next layer.
In view of the above, based on a different-wavelength alignment system (in which illumination light for alignment and light for exposure have different wavelengths) utilizing the TTR technique which has been disclosed in U.S. Pat. No. 4,880,310, for example, there has been proposed a method of optically sensing a one-dimensional diffraction grating mark formed on the wafer or reticle and detecting the position of the wafer or reticle from the resulting pitch information with high resolution (on the order of a fraction to one of several tens fractions of the pitch) in Japanese Patent Laid-Open No. 63-283129 (corresponding to U.S. Ser. No. 192,784 filed on May 10, 1988). A variety of methods have so far been proposed and practiced in detecting the position of the grating mark. Among them, the above method disclosed in Japanese Patent Laid-Open No. 63-283129 is designed to irradiate coherent laser beams (parallel rays) toward the grating mark in two directions simultaneously for forming a one-dimensional interference fringe, and to determine the position of the grating mark from the interference fringe.
The alignment system employing an interference fringe is divided into two categories; i.e., a heterodyne method in which a certain frequency difference is given between two laser beams irradiated in two directions, and a homodyne method which gives no frequency difference. In the homodyne method, a still interference fringe is formed in parallel to a grating mark and the grating mark (or the object) requires to be finely moved in the pitch direction for detecting the position of the grating mark. The grating position is determined on the basis of the interference fringe as a reference. Meanwhile, in the heterodyne method, the frequency difference between the two laser beams (i.e., the beat frequency) causes the interference fringe to flow in the fringe direction (pitch direction) at a high speed. Accordingly, the grating position can be determined not on the basis of the interference fringe, but solely on the basis of a time element (phase difference) incidental to high-speed movement of the interference fringe.
More specifically, by way of example, the heterodyne method determines a phase difference (within .+-.180.degree.) between a photoelectric signal (optical beat signal) detected by modulating the intensity of .+-.1st order diffracted rays from the grating mark with the beat frequency and an optical beat signal of reference interference light separately created from the two transmitted light beams, thereby detecting a position shift within .+-.P/4 of the grating pitch P. Assuming now that the grating pitch P is 2 .mu.m (corresponding to a line and a space being each 1 .mu.m) and measurement of the phase difference has resolution on the order of 0.5.degree., measurement of the position shift exhibits resolution of (P/4).multidot.(0.5/180).perspectiveto.0.0014 .mu.m. Because the mark position detection like this technique provides very high resolution, it is expected that alignment accuracy can be increased one or more orders of level in comparison with the prior-art mark detection.
FIGS. 12A-12C illustrate one example of the schematic configuration of a stepper equipped with an alignment system utilizing the TTR technique. Illumination light for alignment that has a different range of wavelengths from that of light for exposure is converted by a two luminous flux frequency shifter (not shown) into two laser beams BM.sub.1, BM.sub.2 which have different frequencies and each contain both of linearly polarized lights orthogonal to each other. In FIG. 12, above a reticle 74, there is provided a dichroic mirror 73 acting on the exposure light and the two laser beams BM.sub.1, BM.sub.2 separately depending on their wavelengths. A pattern of the reticle 74 is illuminated with the exposure light reflected by the dichroic mirror 73 vertically downward, and focused on a wafer 75 by a projection lens 70 having tele-centric surfaces on both sides. The projection lens 70 is corrected in its chromatic aberration for the wavelength (such as g or i line) of the exposure light. The reticle 74 and the wafer 75 are arranged in conjugate relation to each other under the same wavelength. A window (transparent section) RST is defined in a light shielding band surrounding a pattern area 76 of the reticle 74, and a reticle mark RG is formed in about a half of the window RST. On the other hand, a wafer mark WG is formed at the corresponding position in a street line around each shot area SA on the wafer 75.
An object lens 71 comprises a 2-focus element consisted of a plano-convex lens of double-refractive substance (such as quartz or calcite) and a plano-concave lens of glass with their convex and concave surfaces bonded together, and a tele-centric objective, these two components being integrally combined with each other. The object lens 71 gives different power to the beams BM.sub.1, BM.sub.2 depending on polarized components (one of which parallel to the crystal axis of the 2-focus element is defined as a p-polarized component and the other of which perpendicular thereto is as s-polarized component, by way of example). Accordingly, with one exemplified arrangement, after the beams BM.sub.1, BM.sub.2 emerging from the object lens 71 are reflected by a mirror 72, p-polarized beams contained in both the beams BM.sub.1, BM.sub.2 are focused (crossed) on the reticle mark RG, while s-polarized beams contained therein are once crossed in the focal plane (conjugate to the wafer) located in a space above the reticle 74 and again crossed on the wafer mark WG after passing through the window RST and the projection lens 70 (incident pupil 70a). The spacing between the above focal plane and the lower surface (pattern surface) of the reticle 74 corresponds to an amount of on-axis chromatic aberration caused on the reticle side by the projection lens 70 under the wavelengths of the two beams for alignment. Assuming that the exposure wavelength is 248 nm and the alignment wavelength is 633 nm, for example, the resulting amount of on-axis chromatic aberration amounts to about 500 mm, though depending on optical characteristics of the projection lens 70.
Then, .+-.1st order diffracted rays are produced by each of the reticle mark RG and the wafer mark WG, and their principal rays are returned coaxially with an optical axis AXc of the alignment system and separately received by a photoelectric detector through a spatial filter, a field diaphragm and so forth. The photoelectric detector outputs a photoelectric signal of the interference light on the reticle side and a photoelectric signal of the interference light on the wafer side (both of these photoelectric signals having the beat frequency). A phase difference in the waveform between the two signals is then determined by using as a reference a beat signal of reference interference light separately created from the two transmitted light beams. The reticle 74 and the wafer 75 are moved relatively so that the above phase difference becomes substantially zero, thereby precisely aligning a projected image of the reticle pattern and the shot area SA.
In the above-explained prior art, however, the object lens 71 and the foremost mirror 72 require to be moved together in afocal position and in the plane parallel to the reticle 74 (along the optical axis AXc in the case of FIG. 12) with change in position of the reticle mark depending on the chip size of semiconductor devices, as disclosed in U.S. Pat. No. 4,592,625 by way of example. Therefore, the system becomes so unstable for some reason incidental to movement of the object lens 71 and others that, for example, the optical axis AXc of the alignment system may be inclined in the pitch direction (i.e., the direction of measurement) of the reticle mark RG. Thus, consider now the case where the alignment system is inclined, by referring to FIG. 13. FIG. 13 represents the state in an exaggerated manner where the optical axis AXc is inclined by an angle .epsilon. in the pitch direction (X-direction) about a point Wo in the focal plane (conjugate to the wafer) W' above the reticle 74.
As shown in FIG. 13, because the optical axis AXc is just inclined about the point Wo, principal rays L.sub.1S, L.sub.2S of the two s-polarized beams irradiated onto the wafer mark WG will not be shifted in their cross position on the wafer upon the inclination assumed above. Meanwhile, as to principal rays L.sub.1P, L.sub.2P of the two p-polarized beams irradiated onto the reticle mark RG, their cross position will be shifted on the reticle 74 by .DELTA.l.sub.1 in the X-direction upon the inclination. Stated differently, the cross position of the principal rays L.sub.1P, L.sub.2P and the cross position of the principal rays L.sub.1S, L.sub.2S are shifted by .DELTA.l.sub.1 in the pitch direction. Therefore, the accuracy of detecting a relative shift amount between the reticle and the wafer is deteriorated corresponding to the above shift amount .DELTA.l.sub.1, thus raising a problem that the alignment accuracy is also reduced.
Assuming now that the amount of on-axis chromatic aberration caused on the reticle side by the projection lens under the wavelength of the illumination light for alignment is .DELTA.L, the shift amount .DELTA.l.sub.1 is expressed by .DELTA.l.sub.1 =.DELTA.L.multidot.tan.epsilon.. Accordingly, given the amount of on-axis chromatic aberration being 500 mm and an allowable value (maximum value) of the shift amount .DELTA.l.sub.1 on the reticle from the standpoint of alignment accuracy being 0.05.mu., an allowable angle error (allowable inclination amount) in the alignment system is as very small as .epsilon.=0.02" from the above equation (tan.epsilon.=.DELTA.l.sub.1 /.DELTA.L). This implies that the inclination amount of the overall alignment system must be held within 0.02". But, taking into account instability of the movable alignment system, an inclination on the order of 1-2" will be produced by its very nature in the overall system even if vibrations and thermal fluctuations are minimized. Therefore, the cross position of the principal rays L.sub.1P, L.sub.2P will be shifted at least 2.5 .mu.m on the reticle and thus about 0.5 .mu.m (in the case of the projection lens with a projecting magnification of 1/5) on the wafer. As a result, even a high-resolution alignment system which has practical measurement resolution as high as 0.01 .mu.m in consideration of an effect of noise and so forth becomes ineffective in practical use. It is to be noted that other than vibration of the alignment system, drifts of the beams BM.sub.1, BM.sub.2 caused by fluctuations in position of a pair of oscillation mirrors disposed in a laser light source, for example, may also reduce the alignment accuracy.
Furthermore, when the object lens 71 (only the telecentric objective) is offset from the optical axis AXc (or AX') of the alignment system, for example, the cross position of the principal rays L.sub.1S, L.sub.2S on the wafer (or a conjugate surface W') and the cross position of the principal rays L.sub.1P, L.sub.2P on the reticle are also shifted by .DELTA.l.sub.2 in the X-direction (pitch direction). This shift amount .DELTA.l.sub.2 directly leads to an alignment error in a like manner. Consequently, the alignment system using illumination light having a different range of wavelengths from exposure light has suffered from the problem that if an alignment beam is inclined with respect to the direction of measurement of alignment marks (i.e., in the pitch direction) due to such as inclination of its optical system or offset of optical components, the alignment accuracy is remarkably reduced by the presence of the on-axis chromatic aberration .DELTA.L.